Network Working Group S. Floyd
Request for Comments: 2582 ACIRI
Category: Experimental T. Henderson
U.C. Berkeley
April 1999
The NewReno Modification to TCP's Fast Recovery Algorithm
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
RFC 2001 [RFC2001] documents the following four intertwined TCP
congestion control algorithms: Slow Start, Congestion Avoidance, Fast
Retransmit, and Fast Recovery. RFC 2581 [RFC2581] explicitly allows
certain modifications of these algorithms, including modifications
that use the TCP Selective Acknowledgement (SACK) option [MMFR96],
and modifications that respond to "partial acknowledgments" (ACKs
which cover new data, but not all the data outstanding when loss was
detected) in the absence of SACK. This document describes a specific
algorithm for responding to partial acknowledgments, referred to as
NewReno. This response to partial acknowledgments was first proposed
by Janey Hoe in [Hoe95].
1. Introduction
For the typical implementation of the TCP Fast Recovery algorithm
described in [RFC2581] (first implemented in the 1990 BSD Reno
release, and referred to as the Reno algorithm in [FF96]), the TCP
data sender only retransmits a packet after a retransmit timeout has
occurred, or after three duplicate acknowledgements have arrived
triggering the Fast Retransmit algorithm. A single retransmit
timeout might result in the retransmission of several data packets,
but each invocation of the Reno Fast Retransmit algorithm leads to
the retransmission of only a single data packet.
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
Problems can arise, therefore, when multiple packets have been
dropped from a single window of data and the Fast Retransmit and Fast
Recovery algorithms are invoked. In this case, if the SACK option is
available, the TCP sender has the information to make intelligent
decisions about which packets to retransmit and which packets not to
retransmit during Fast Recovery. This document applies only for TCP
connections that are unable to use the TCP Selective Acknowledgement
(SACK) option.
In the absence of SACK, there is little information available to the
TCP sender in making retransmission decisions during Fast Recovery.
From the three duplicate acknowledgements, the sender infers a packet
loss, and retransmits the indicated packet. After this, the data
sender could receive additional duplicate acknowledgements, as the
data receiver acknowledges additional data packets that were already
in flight when the sender entered Fast Retransmit.
In the case of multiple packets dropped from a single window of data,
the first new information available to the sender comes when the
sender receives an acknowledgement for the retransmitted packet (that
is the packet retransmitted when Fast Retransmit was first entered).
If there had been a single packet drop, then the acknowledgement for
this packet will acknowledge all of the packets transmitted before
Fast Retransmit was entered (in the absence of reordering). However,
when there were multiple packet drops, then the acknowledgement for
the retransmitted packet will acknowledge some but not all of the
packets transmitted before the Fast Retransmit. We call this packet
a partial acknowledgment.
Along with several other suggestions, [Hoe95] suggested that during
Fast Recovery the TCP data sender respond to a partial acknowledgment
by inferring that the indicated packet has been lost, and
retransmitting that packet. This document describes a modification
to the Fast Recovery algorithm in Reno TCP that incorporates a
response to partial acknowledgements received during Fast Recovery.
We call this modified Fast Recovery algorithm NewReno, because it is
a slight but significant variation of the basic Reno algorithm. This
document does not discuss the other suggestions in [Hoe95] and
[Hoe96], such as a change to the ssthresh parameter during Slow-
Start, or the proposal to send a new packet for every two duplicate
acknowledgements during Fast Recovery. The version of NewReno in
this document also draws on other discussions of NewReno in the
literature [LM97].
We do not claim that the NewReno version of Fast Recovery described
here is an optimal modification of Fast Recovery for responding to
partial acknowledgements, for TCPs that are unable to use SACK.
Based on our experiences with the NewReno modification in the NS
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
simulator [NS], we believe that this modification improves the
performance of the Fast Retransmit and Fast Recovery algorithms in a
wide variety of scenarios, and we are simply documenting it for the
benefit of the IETF community. We encourage the use of this
modification to Fast Recovery, and we further encourage feedback
about operational experiences with this or related modifications.
2. Definitions
This document assumes that the reader is familiar with the terms
MAXIMUM SEGMENT SIZE (MSS), CONGESTION WINDOW (cwnd), and FLIGHT SIZE
(FlightSize) defined in [RFC2581]. FLIGHT SIZE is defined as in
[RFC2581] as follows:
FLIGHT SIZE:
The amount of data that has been sent but not yet acknowledged.
3. The Fast Retransmit and Fast Recovery algorithms in NewReno
The standard implementation of the Fast Retransmit and Fast Recovery
algorithms is given in [RFC2581]. The NewReno modification of these
algorithms is given below. This NewReno modification differs from
the implementation in [RFC2581] only in the introduction of the
variable "recover" in step 1, and in the response to a partial or new
acknowledgement in step 5. The modification defines a "Fast Recovery
procedure" that begins when three duplicate ACKs are received and
ends when either a retransmission timeout occurs or an ACK arrives
that acknowledges all of the data up to and including the data that
was outstanding when the Fast Recovery procedure began.
1. When the third duplicate ACK is received and the sender is not
already in the Fast Recovery procedure, set ssthresh to no more
than the value given in equation 1 below. (This is equation 3
from [RFC2581]).
ssthresh = max (FlightSize / 2, 2*MSS) (1)
Record the highest sequence number transmitted in the variable
"recover".
2. Retransmit the lost segment and set cwnd to ssthresh plus 3*MSS.
This artificially "inflates" the congestion window by the number
of segments (three) that have left the network and which the
receiver has buffered.
3. For each additional duplicate ACK received, increment cwnd by
MSS. This artificially inflates the congestion window in order
to reflect the additional segment that has left the network.
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
4. Transmit a segment, if allowed by the new value of cwnd and the
receiver's advertised window.
5. When an ACK arrives that acknowledges new data, this ACK could be
the acknowledgment elicited by the retransmission from step 2, or
elicited by a later retransmission.
If this ACK acknowledges all of the data up to and including
"recover", then the ACK acknowledges all the intermediate
segments sent between the original transmission of the lost
segment and the receipt of the third duplicate ACK. Set cwnd to
either (1) min (ssthresh, FlightSize + MSS); or (2) ssthresh,
where ssthresh is the value set in step 1; this is termed
"deflating" the window. (We note that "FlightSize" in step 1
referred to the amount of data outstanding in step 1, when Fast
Recovery was entered, while "FlightSize" in step 5 refers to the
amount of data outstanding in step 5, when Fast Recovery is
exited.) If the second option is selected, the implementation
should take measures to avoid a possible burst of data, in case
the amount of data outstanding in the network was much less than
the new congestion window allows [HTH98]. Exit the Fast Recovery
procedure.
If this ACK does *not* acknowledge all of the data up to and
including "recover", then this is a partial ACK. In this case,
retransmit the first unacknowledged segment. Deflate the
congestion window by the amount of new data acknowledged, then
add back one MSS and send a new segment if permitted by the new
value of cwnd. This "partial window deflation" attempts to
ensure that, when Fast Recovery eventually ends, approximately
ssthresh amount of data will be outstanding in the network. Do
not exit the Fast Recovery procedure (i.e., if any duplicate ACKs
subsequently arrive, execute Steps 3 and 4 above).
For the first partial ACK that arrives during Fast Recovery, also
reset the retransmit timer.
Note that in Step 5, the congestion window is deflated when a partial
acknowledgement is received. The congestion window was likely to
have been inflated considerably when the partial acknowledgement was
received. In addition, depending on the original pattern of packet
losses, the partial acknowledgement might acknowledge nearly a window
of data. In this case, if the congestion window was not deflated,
the data sender might be able to send nearly a window of data back-
to-back.
There are several possible variants to the simple response to partial
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
acknowledgements described above. First, there is a question of when
to reset the retransmit timer after a partial acknowledgement. This
is discussed further in Section 4 below.
There is a related question of how many packets to retransmit after
each partial acknowledgement. The algorithm described above
retransmits a single packet after each partial acknowledgement. This
is the most conservative alternative, in that it is the least likely
to result in an unnecessarily-retransmitted packet. A variant that
would recover faster from a window with many packet drops would be to
effectively Slow-Start, requiring less than N roundtrip times to
recover from N losses [Hoe96]. With this slightly-more-aggressive
response to partial acknowledgements, it would be advantageous to
reset the retransmit timer after each retransmission. Because we
have not experimented with this variant in our simulator, we do not
discuss this variant further in this document.
A third question involves avoiding multiple Fast Retransmits caused
by the retransmission of packets already received by the receiver.
This is discussed in Section 5 below. Avoiding multiple Fast
Retransmits is particularly important if more aggressive responses to
partial acknowledgements are implemented, because in this case the
sender is more likely to retransmit packets already received by the
receiver.
As a final note, we would observe that in the absence of the SACK
option, the data sender is working from limited information. One
could spend a great deal of time considering exactly which variant of
Fast Recovery is optimal for which scenario in this case. When the
issue of recovery from multiple dropped packets from a single window
of data is of particular importance, the best alternative would be to
use the SACK option.
4. Resetting the retransmit timer.
The algorithm in Section 3 resets the retransmit timer only after the
first partial ACK. In this case, if a large number of packets were
dropped from a window of data, the TCP data sender's retransmit timer
will ultimately expire, and the TCP data sender will invoke Slow-
Start. (This is illustrated on page 12 of [F98].) We call this the
Impatient variant of NewReno.
In contrast, the NewReno simulations in [FF96] illustrate the
algorithm described above, with the modification that the retransmit
timer is reset after each partial acknowledgement. We call this the
Slow-but-Steady variant of NewReno. In this case, for a window with
a large number of packet drops, the TCP data sender retransmits at
most one packet per roundtrip time. (This behavior is illustrated in
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
the New-Reno TCP simulation of Figure 5 in [FF96], and on page 11 of
[F98].)
For TCP implementations where the Retransmission Timeout Value (RTO)
is generally not much larger than the round-trip time (RTT), the
Impatient variant can result in a retransmit timeout even in a
scenario with a small number of packet drops. For TCP
implementations where the Retransmission Timeout Value (RTO) is
usually considerably larger than the round-trip time (RTT), the Slow-
but-Steady variant can remain in Fast Recovery for a long time when
multiple packets have been dropped from a window of data. Neither of
these variants are optimal; one possibility for a more optimal
algorithm might be one that recovered more quickly from multiple
packet drops, and combined this with the Slow-but-Steady variant in
terms of resetting the retransmit timers. We note, however, that
there is a limitation to the potential performance in this case in
the absence of the SACK option.
5. Avoiding Multiple Fast Retransmits
In the absence of the SACK option, a duplicate acknowledgement
carries no information to identify the data packet or packets at the
TCP data receiver that triggered that duplicate acknowledgement. The
TCP data sender is unable to distinguish between a duplicate
acknowledgement that results from a lost or delayed data packet, and
a duplicate acknowledgement that results from the sender's
retransmission of a data packet that had already been received at the
TCP data receiver. Because of this, multiple segment losses from a
single window of data can sometimes result in unnecessary multiple
Fast Retransmits (and multiple reductions of the congestion window)
[Flo94].
With the Fast Retransmit and Fast Recovery algorithms in Reno or
NewReno TCP, the performance problems caused by multiple Fast
Retransmits are relatively minor (compared to the potential problems
with Tahoe TCP, which does not implement Fast Recovery).
Nevertheless, unnecessary Fast Retransmits can occur with Reno or
NewReno TCP, particularly if a Retransmit Timeout occurs during Fast
Recovery. (This is illustrated for Reno on page 6 of [F98], and for
NewReno on page 8 of [F98].) With NewReno, the data sender remains
in Fast Recovery until either a Retransmit Timeout, or until all of
the data outstanding when Fast Retransmit was entered has been
acknowledged. Thus with NewReno, the problem of multiple Fast
Retransmits from a single window of data can only occur after a
Retransmit Timeout.
The following modification to the algorithms in Section 3 eliminates
the problem of multiple Fast Retransmits. (This modification is
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
called "bugfix" in [F98], and is illustrated on pages 7 and 9.) This
modification uses a new variable "send_high", whose initial value is
the initial send sequence number. After each retransmit timeout, the
highest sequence numbers transmitted so far is recorded in the
variable "send_high".
If, after a retransmit timeout, the TCP data sender retransmits three
consecutive packets that have already been received by the data
receiver, then the TCP data sender will receive three duplicate
acknowledgements that do not acknowledge "send_high". In this case,
the duplicate acknowledgements are not an indication of a new
instance of congestion. They are simply an indication that the
sender has unnecessarily retransmitted at least three packets.
We note that if the TCP data sender receives three duplicate
acknowledgements that do not acknowledge "send_high", the sender does
not know whether these duplicate acknowledgements resulted from a new
packet drop or not. For a TCP that implements the bugfix described
in this section for avoiding multiple fast retransmits, the sender
does not infer a packet drop from duplicate acknowledgements in these
circumstances. As always, the retransmit timer is the backup
mechanism for inferring packet loss in this case.
The modification to Fast Retransmit for avoiding multiple Fast
Retransmits replaces Step 1 in Section 3 with Step 1A below. In
addition, the modification adds Step 6 below:
1A. When the third duplicate ACK is received and the sender is not
already in the Fast Recovery procedure, check to see if those
duplicate ACKs cover more than "send_high". If they do, then set
ssthresh to no more than the value given in equation 1, record
the the highest sequence number transmitted in the variable
"recover", and go to Step 2. If the duplicate ACKs don't cover
"send_high", then do nothing. That is, do not enter the Fast
Retransmit and Fast Recovery procedure, do not change ssthresh,
do not go to Step 2 to retransmit the "lost" segment, and do not
execute Step 3 upon subsequent duplicate ACKs.
Steps 2-5 are the same as those steps in Section 3 above.
6. After a retransmit timeout, record the highest sequence number
transmitted in the variable "send_high" and exit the Fast
Recovery procedure if applicable.
Step 1A above, in checking whether the duplicate ACKs cover *more*
than "send_high", is the Careful variant of this algorithm. Another
possible variant would be to require simply that the three duplicate
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
acknowledgements *cover* "send_high" before initiating another Fast
Retransmit. We call this the Less Careful variant to Fast
Retransmit.
There are two separate scenarios in which the TCP sender could
receive three duplicate acknowledgements acknowledging "send_high"
but no more than "send_high". One scenario would be that the data
sender transmitted four packets with sequence numbers higher than
"send_high", that the first packet was dropped in the network, and
the following three packets triggered three duplicate
acknowledgements acknowledging "send_high". The second scenario
would be that the sender unnecessarily retransmitted three packets
below "send_high", and that these three packets triggered three
duplicate acknowledgements acknowledging "send_high". In the absence
of SACK, the TCP sender in unable to distinguish between these two
scenarios.
For the Careful variant of Fast Retransmit, the data sender would
have to wait for a retransmit timeout in the first scenario, but
would not have an unnecessary Fast Retransmit in the second scenario.
For the Less Careful variant to Fast Retransmit, the data sender
would Fast Retransmit as desired in the first scenario, and would
unnecessarily Fast Retransmit in the second scenario. The NS
simulator has implemented the Less Careful variant of NewReno, and
the TCP implementation in Sun's Solaris 7 implements the Careful
variant. This document recommends the Careful variant given in Step
1A above.
6. Implementation issues for the data receiver.
[RFC2001] specifies that "Out-of-order data segments SHOULD be
acknowledged immediately, in order to trigger the fast retransmit
algorithm." Neal Cardwell has noted [C98] that some data receivers do
not send an immediate acknowledgement when they send a partial
acknowledgment, but instead wait first for their delayed
acknowledgement timer to expire. As [C98] notes, this severely
limits the potential benefit from NewReno by delaying the receipt of
the partial acknowledgement at the data sender. Our recommendation
is that the data receiver send an immediate acknowledgement for an
out-of-order segment, even when that out-of-order segment fills a
hole in the buffer.
7. Simulations
Simulations with NewReno are illustrated with the validation test
"tcl/test/test-all-newreno" in the NS simulator. The command
"../../ns test-suite-newreno.tcl reno" shows a simulation with Reno
TCP, illustrating the data sender's lack of response to a partial
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 1999
acknowledgement. In contrast, the command "../../ns test-suite-
newreno.tcl newreno_B" shows a simulation with the same scenario
using the NewReno algorithms described in this paper.
The tests "../../ns test-suite-newreno.tcl newreno1_B0" and "../../ns
test-suite-newreno.tcl newreno1_B" show the Slow-but-Steady and the
Impatient variants of NewReno, respectively.
8. Conclusions
Our recommendation is that TCP implementations include the NewReno
modification to the Fast Recovery algorithm given in Section 3, along
with the modification for avoiding multiple Fast Retransmits given in
Section 5. The NewReno modification given in Section 3 can be
important even for TCP implementations that support the SACK option,
because the SACK option can only be used for TCP connections when
both TCP end-nodes support the SACK option. The NewReno modification
given in Section 3 implements the Impatient rather than the Slow-but-
Steady variant of NewReno.
While this document mentions several possible variations to the
NewReno algorithm, we have not explored all of these possible
variations, and therefore are unable to make recommendations about
some of them. Our belief is that the differences between any two
variants of NewReno are small compared to the differences between
Reno and NewReno. That is, the important thing is to implement
NewReno instead of Reno, for a TCP invocation without SACK; it is
less important exactly which variant of NewReno is implemented.
9. Acknowledgements
Many thanks to Anil Agarwal, Mark Allman, Vern Paxson, Kacheong Poon,
and Bernie Volz for detailed feedback on this document.
10. References
[C98] Neal Cardwell, "delayed ACKs for retransmitted packets:
ouch!". November 1998. Email to the tcpimpl mailing
list, Message-ID "Pine.LNX.4.02A.9811021421340.26785-
100000@sake.cs.washington.edu", archived at
"http://tcp-impl.lerc.nasa.gov/tcp-impl".
[F98] Sally Floyd. Revisions to RFC 2001. Presentation to
the TCPIMPL Working Group, August 1998. URLs
"ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.ps" and
"ftp://ftp.ee.lbl.gov/talks/sf-tcpimpl-aug98.pdf".
[FF96] Kevin Fall and Sally Floyd. Simulation-based
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RFC 2582 NewReno Modification to TCP's Fast Recovery April 199913. Full Copyright Statement
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